The present invention concerns a procedure and apparatus based on X-ray diffraction for measuring stress states in metals. More particularly, the invention relates to a procedure for measuring stress states in austenitic steels or the equivalent. In the apparatus of the invention, a detector means is inclined about an axis lying on the surface of the specimen under examination which is substantially perpendicular to the orientation of the stresses to be studied. By mediation of the detector means, the diameters parallel to the surface under examination of the so-called Debye rings, or the equivalent, are recorded at two or more angles of inclination. The detector, as viewed in the direction at right angles against the inclination axis, has an arcuate shape. The detector means is elongated in the direction of the inclination axis and narrow enough in the opposite direction to make feasible an inclination angle of the detector means great enough to carry out the invention.
The invention further concerns a measuring instrument for carrying out the procedure, comprising an X-ray tube disposed together with its ancillary apparatus to be rotated about the inclination axis, said axis passing through the point to be examined of the specimen to be examined. As viewed in a direction at right angles with the inclination axis, the detector means is an arcuate shape, preferably that of a circular arc.
The need for non-destructive stress measurement has increased considerably in recent years. A non-destructive stress measuring method in more extensive use is based on the diffraction of X-rays in crystalline matter. The greatest shortcoming of the measuring equipment is its complex design and great bulk, impeding its application in field conditions.
Stress measuring methods based on X-ray diffraction measure the stresses in the surface of crystalline material. The depth of penetration of the X-rays is on the order of 5 to 24 .mu.m. The stresses in the surface are significant with a view to the durability of structures, because damage usually starts in the superficial layers, as exemplified by stress corrosion, brittle fracture, fatigue, etc.
According to international estimates, the annual losses from material-technological faults amount to about 50 billion Finnish marks, a great part thereof caused by residual stresses.
Residual stresses always arise as a result of inhomogeneous deformation, which may be a consequence of the following factors, among others: working, temperature differences, phase transformation, different thermal expansion coefficients of different phases. Residual stresses are incurred in metal treatments such as, for example, welding, heat treatments and machining, for example, by grinding.
Residual stresses are usually divided into two groups depending on their distance of influence. These groups are macrostresses and microstresses. The sphere of influence of macrostresses extends, at a minimum, over several grains, whereas microstresses are concentrated in the region of one grain. Macrostresses are measured by X-ray diffraction, and are either compressive or tensile residual stresses. Tensile stresses are unfavorable in the surface from the viewpoint of structures. On the other hand, compressive stresses improve, for example, the fatigue strength of metals, and it is therefore often desirable to achieve a compressive stress in the surface of metallic components. The most important methods by which a state of compressive stress can be produced are ball blasting, rolling, hammering, and in general any method by which a metal surface is plastically deformed.
There has been comparatively little practical application of non-destructive stress measuring methods and means of the prior art.
Finnish Pat. No. 61248 discloses an improved camera procedure utilizing non-bulk apparatus of simple design, compared with diffractometer apparatus that was in general use theretofore, to eliminate the aforementioned disadvantages.
There are in principle two types of stress measuring instruments based on X-ray diffraction. These types are camera and diffractometer apparatus. The simplest and smallest instrument construction is achieved in the camera construction, but its drawback is the comparatively long time required to expose and develop the film. With diffractometers, the X-ray intensity is measured as a function of the 2.theta. angle with a proportional or scintillation counter. A scan must be run with the counter over a given 2.theta. angle range, and this requires an accurate goniometer apparatus. For this reason, diffractometers are relatively bulky, complex and expensive instruments as far as field work is concerned. It has been possible somewhat to reduce the size of diffractometers and to increase their speed with the aid of so-called location-sending detectors.
Since the use of the camera method disclosed in Finnish Pat. No. 61248 is associated with a comparatively long time required for exposing and developing the film, the object of the present invention is to develop a stress measuring procedure and apparatus which has a bulk on the same order as the camera construction, but which is substantially faster in use.
In order to attain the aims stated, and others which will become apparent later on, the invention utilizes a detector means by which the X-rays reflected from the specimen are converted into photosignals, and the stresses to be measured are determined from these photosignals.